The present invention relates technique of air-fuel ratio control for purification of exhaust gases from an engine.
For efficient and simultaneous purification of noxious emissions HC, CO and NOx, a three way catalyst calls for an atmosphere of a stoichiometric air-fuel ratio. A catalyst having a capability of oxygen storage can keep such a stoichiometric atmosphere of stoichiometric oxygen concentration by absorbing an excess of oxygen in an exhaust gas mixture flowing into the catalyst and releasing oxygen corresponding to an excess of reducing agents (HC, CO) in the exhaust gas mixture. When a lean exhaust gas mixture leaner than the stoichiometry flows into the catalyst, the catalyst absorbs an excess of oxygen instantly and maintains the stoichiometric atmosphere until the oxygen storage amount of the catalyst reaches saturation. When a rich exhaust gas mixture richer than the stoichiometry flows into the catalyst, the catalyst desorbs oxygen instantly to remedy the deficiency in oxygen and maintain the stoichiometric atmosphere until the stored oxygen is fully desorbed.
Thus, the oxygen storage type catalyst can hold its atmosphere at the stoichiometric state by compensating for any excess or deficiency of oxygen caused by temporary air-fuel ratio deviations. However, in the saturated state in which the oxygen storage amount reaches a saturation level or in the empty state in which the catalyst has no stored oxygen, the catalyst cannot efficiently purify HC, CO and NOx any more, so that the exhaust emission degrades.
Japanese Patent Kokai Publications No. H5(1993)-195842 and No. H7(1995)-259602 propose feedback control systems for controlling an oxygen storage amount of a catalyst to prevent degradation of exhaust emission.